Light emitting diode digital micromirror device illuminator

ABSTRACT

Described are optical systems for a digital micromirror device (DMD) illuminator. The optical systems include a LED array, a tapered non-imaging collection optic, a reflective stop and a telecentric lens system. The telecentric lens system is disposed along an optical axis defined between the tapered non-imaging collection optic and the reflective stop. The telecentric lens system is configured as a first half of a symmetric one to one imager for an object plane on the optical axis and as a second half of the symmetric one to one imager for optical energy reflected from the reflective aperture stop. The optical systems reclaim optical energy emitted by the LED array that does not initially pass through the reflective stop and provide an improved intensity distribution at the DMD. Reductions in stray light and the thermal loads on the illuminator and DMD are achieved relative to conventional illumination systems for DMDs.

RELATED APPLICATION

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 14/705,202, filed May 6, 2015 and titled “LightEmitting Diode Digital Micromirror Device Illuminator,” which claims thebenefit of and priority to U.S. Provisional Patent Application No.61/991,488, filed May 10, 2014 and titled “High Radiance UltravioletLight Emitting Diode Digital Micromirror Device Illuminator.” Thecontents of these applications are expressly incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

The invention relates to high radiance Ultraviolet (UV) sources ofillumination coupled to projection systems for selectively exposingphotocurable materials in applications such as maskless lithography andtwo and three dimensional digital printing.

BACKGROUND

High brightness light emitting diode (LED) light sources have onlyrecently become available at performance levels suitable for challenginghigh intensity applications in UV curing. There has been a significantincrease in the use and value of UV cured materials in the manufacturingprocess due to significantly higher production throughput afforded bythe extremely fast curing of materials in comparison to curing by otherconventional means including heat, non-photoinitiated chemicalinteractions of adhesives, evaporation of volatiles, and the like. Thisimprovement in process time has significant value to the manufacturingcommunity. Imaged UV curing in the case of maskless lithography savesconsiderable cost in eliminating the need to produce the mask, inaddition to the time savings. In the case of three dimensional (3-D)printing, faster cure times result in faster build times which whencombined with new high performance curable UV resins enables true 3-Ddigital printing for manufacturing, an area of technology which isgrowing quickly.

Conventional UV curing devices typically utilize short arc mercurylamps, xenon lamps, lasers and more recently, systems incorporatingpre-packaged high brightness LEDs. Conventional arc lamps suffer fromsignificant arc flicker resulting in the need to homogenize the lightwhich in typical non-Etendue preserving designs reduces radiance(optical power per unit area per unit solid angle [W/cm²/sr]). Arc lampsalso suffer from poor lifetime and rapidly decreasing output as afunction of time with lifetimes in the hundreds to low thousands ofhours at best. This results in added system cost and maintenance expenserelative to LED sources which are more stable in output in both theshort and long term, and characterized by lifetimes of tens of thousandsof hours given proper attention to thermal design. In the last severalyears laser diodes have been used to replace arc lamps, however, laserdiodes are extremely expensive compared to both arc lamps and LEDs, andsuffer from image artifacts due to the high temporal and spatialcoherence relative to LEDs. Conventional implementations using LEDs,however, suffer from relatively low radiance and lifetime due tolimitations of their optical, mechanical and thermal designs.

Prepackaged LEDs are defined as devices comprising an LED die or diearray sitting on top of one or more thermally and electricallyconductive materials. The thermal impedances of these multipleinterfaces leads to a cumulative total thermal impedance that results inhigh LED junction temperature, thereby degrading LED output and life.Examples of prepackaged UV LED devices include those offered by theSemiLeds® and Nichia® product lines.

SUMMARY

A UV LED digital micromirror device (DMD) illuminator according toembodiments of the invention uses a three way telecentric opticalimaging system, telecentric in object and image space as well as at theaperture stop, to enhance the intensity imaged from the LED arraythrough the tapered non-imaging collection optic, through a keystonecorrected and tilted hollow light integrator, with integralparallelogram shaped field stop and a telecentric stop which reimageslight that falls outside the dimensions of the aperture stop, and issized to match the 12 degree half angle of typical DMD devices. Thecombination of the reclaimed light that would otherwise be lost at theaperture stop together with the optimal imaging to just overfill theDMD's active area results in optimal efficiency, reduced thermal load onthe illuminator housing and on the DMD device and reduced stray lightfor optimal image contrast in comparison with other commerciallyavailable UV DMD illumination systems.

Illuminators, according to embodiments of the invention, provide ahighly reliable and high power density source of uniform illuminationfor use in a variety of applications including maskless lithography,selective curing of printed inks and 3-D printing.

Embodiments of the invention described herein include a high radiance UVLED illuminator that projects onto a DMD as a source of high radiance UVenergy to be subsequently imaged by a well corrected projection lenswith minimal distortion onto an illumination plane with a high degree ofspatial uniformity and high intensity. Applications include acting asthe illumination portion of a system designed for maskless lithographyand 3-D printing and other related UV cured materials applications or asa part of a system providing structured fluorescence excitation.

In one aspect, an optical system for a digital micromirror deviceilluminator includes a LED array, a tapered non-imaging collectionoptic, a reflective aperture stop and a telecentric lens system. Thetapered non-imaging collection optic has an input aperture and an outputaperture. The input aperture is in optical communication with the LEDarray to receive optical energy emitted by the LED array. Thetelecentric lens system is disposed between the tapered non-imagingcollection optic and the reflective aperture stop along an optical axisdefined between the LED array and the reflective aperture stop. Thetelecentric lens system is configured as a first half of a symmetric oneto one imager for an object plane on the optical axis and as a secondhalf of the symmetric one to one imager for optical energy reflectedfrom the reflective aperture stop toward an image plane on the opticalaxis.

In another aspect, a hollow light integrator includes a hollow body andan optical field stop. The hollow body has an input aperture at a firstend, an output aperture at a second end opposite to the first end and aplurality of inside walls extending along an axis from the inputaperture to the output aperture. Each of the inside walls includes areflective surface. The first end includes an input face that is normalto the axis and the second end includes an output face that is tiltedwith respect to the axis. The optical field stop is shaped as aparallelogram.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and methodology of the invention, togetherwith other objects and advantages thereof, may best be understood byreading the following detailed description in connection with thedrawings in which each part has an assigned numeral or label thatidentifies it wherever it appears in the various drawings and wherein:

FIG. 1 is a diagrammatic isometric view of a preferred embodiment of ahigh radiance UV LED illuminator comprising LED Board, heat exchanger,and lens assembly. The relative position and size of the illuminationimage are shown.

FIG. 2 is a diagrammatic isometric view of the system of FIG. 1indicating additional detail.

FIG. 3 is a diagrammatic cross sectional view of the system of FIG. 1showing internal optical and mechanical components as well as the LEDarray interfaced to the non-imaging collection optic.

FIGS. 4A and 4B show, respectively, a diagrammatic isometric view and across sectional view of the LED board and collection optic of the systemof FIG. 1.

FIGS. 5A and 5B show, respectively, a diagrammatic isometric view of thesystem of FIG. 4 with the collection optic removed and a close up of theLED array and heat spreader.

FIGS. 6A and 6B show, respectively, a top view and side view of theoptical system of FIG. 3 indicating the rays as transmitted out of thecollection optic aperture and onto the DMD illumination plane.

FIGS. 7A, 7B and 7C show, respectively, a diagrammatic isometric view ofan alternative embodiment of the collection optic of FIG. 3 withreflective aperture, and top and side views showing how rays passthrough the output aperture and reflect off the internal mirroredaperture of the collection optic and back through the exit aperture.

FIG. 8A is a diagrammatic view of another embodiment of a high radianceUV LED illuminator.

FIG. 8B is a diagrammatic isometric view of the system of FIG. 8Aindicating a ray that is emitted from the LED array.

FIG. 8C is an image of the intensity profile of the light that imagesoutside of the optical system's apertures stop that is incident on themirrored portion of the aperture stop.

FIGS. 9A and 9B show, respectively, a system comprising a diagrammaticview of the system of FIG. 8A with the total internal reflection (TIR)element replaced by actual TIR elements and a diagrammatic isometricview detail of the tapered collection optic and hollow light integratorof FIG. 9A.

FIGS. 10A, 10B, 10C and 10D show, respectively, a diagrammatic isometricview of the light tunnel of FIG. 9A, a side view showing the tiltedobject plane, a top view indicating a reflective rim which would becoincident with the tapered collection optic of FIG. 9B, and a top viewof the output side.

FIGS. 10E, 10F, 10G and 10H show, respectively, a diagrammatic isometricview of the light tunnel similar to that of FIG. 9A but with the outputobject face plane having no tilt relative to its input face, a side viewshowing the non-tilted object plane, a top view indicating a reflectiverim which would be coincident with the tapered collection optic of FIG.9B, and a top view of the output side.

FIGS. 11A, 11C and 11E show, respectively, a diagrammatic output sidetop views and side view of a hollow light integrator with no tilt and nofield stop, a light integrator with tilt and no field stop and a lightintegrator with tilt and a parallelogram-shaped field stop.

FIGS. 11B, 11D and 11F show the resulting images on the DMD plane forthe light integrators of FIGS. 11A, 11C and 11E, respectively.

DETAILED DESCRIPTION

The present invention relates to LED illumination used for thephotopolymerization of materials with controlled spatial structure,i.e., imaged UV energy, afforded by imaging the output of a high powerUV LED array onto the aperture of a DMD and subsequently through aprojection lens and onto a desired surface of illumination.

Embodiments of the present invention include an LED based illuminationsource for improved intensity and spatial uniformity at the illuminationplane resulting in higher cured product throughput which in turndecreases the cost of manufacturing processes relative to prior art. Thepresent invention is distinguished from conventional illuminationsources in that it comprises a high radiance profile with high spatialuniformity which optimizes system performance and results in muchimproved lifetime due to minimizing thermal impedance between the bottomsides of the LED die and associated heat sinking elements. Furtherenhancement of intensity results from reclaiming a considerable fractionof the optical power that would otherwise be lost at the aperture stopof the imaging system and redirecting it onto the DMD device. Additionalsystem level improvements of the invention result from decreasing theoptical power on the DMD outside its active area, thereby decreasingthermal load and increasing lifetime of the DMD element itself anddecreased stray light, thereby improving image contrast.

Referring now to FIG. 1, there is shown a diagrammatic, isometric viewof the outside of a preferred embodiment of the UV LED DMD illuminationsource designated generally as system 10. The UV LED illumination sourcesystem 10 comprises an LED assembly 12, a lens housing 14, and a lenssystem of which the most distal lens element 16 is shown. Theillumination device projects an image 18 along the Z-axis which ispositioned coincident with a DMD device with similar aperture extentsand comprising telecentric illumination at each position on the DMDwithin a 12° half angle of acceptance. The long axis of the rectangularDMD aperture is oriented along the horizontal X-axis and the short axisof the aperture in the vertical Y-axis. Typically the size of theilluminated image from the system 10 overfills the DMD slightly, forexample by 5% to 10% to account for system positional tolerances.

Referring now to FIG. 2, there is shown a diagrammatic isometric view 20of the illumination source 10 of FIG. 1 showing additional structure ofthe illumination assembly. LED board assembly 12 is sandwiched betweenthe flange of the lens housing 14 and the water heat exchanger assembly30 by three bolts 32 positioned symmetrically about the flange at 120degree intervals to apply uniform pressure of the back side of the LEDboard 22 and the water heat exchanger 30. A thin uniform layer of highthermal conductivity material such as silver conductive grease, aluminumoxide thermal grease, phase change thermal gap filling material, solder,diamond thermal paste, etc. is deposited between the back side of copperLED board 22 and heat exchanger face of heat exchanger 30 to optimallycouple heat between the devices. In a preferred embodiment silver filledthermal grease is used. One eight-pin high current connector 24 is usedfor the common anode connection. An additional eight-pin connector 26 isused for the cathode; however, it is divided into four pins per 2channels to allow for use of 2 different spectral bins of UV LED die.This can readily be extended up to the total number of LED die in thearray, if desired. There is also a photosensor and thermistor attachedto the LED board which is connected by a smaller, low current connector28. The front flange, surrounding distal lens 16, has a pair ofkinematic interface features 36 comprising a hole and a slot to assurethat the illumination system can be accurately and repeatedly attachedto the mating DMD assembly (not shown) by four bolts going through fourholes 34 on the distal flange of the lens housing 14.

Referring now to FIG. 3, there is shown a diagrammatic cross section ofan isometric view of the system 20 of FIG. 2. With reference from rightto left, the water heat exchanger 30 can have microchannel features toallow water to flow with optimal exchange of heat between the coppermaterial it is made of and the water flowing between the heat exchangerinlet and outlet at a given flow rate. The back of the LED board 12comprising a sheet of copper 22 is thermal interfaced to the heatexchanger 30 with silver filled thermal paste. The LED die array 42 isshown interfacing proximally to a tapered straight sided non-imagingcollection optic 52 within its rectangular aperture, the details ofwhich will be made clear below. The collection optic 52, commonlyreferred to as a taper, is centered and held in position relative to LEDarray 42 by holders 44 and 50 with spring 48 pushing the taper upagainst first lens element 54. A spacer 56 positions second lens element58 up against the lens systems aperture stop 60 which also functions asa spacer for third lens element 64. Lens element 64 makes direct contactnear its outer edge with distal lens element 16, thereby requiring nospacer. A lens retainer 66 holds the four lens elements and two spacerssecurely up against the seat in the housing 14 of lens 54. This resiststhe spring 48 from pushing the lens 54 toward the output of the lenssystems and assures that the taper 52 is reliably positioned in distanceby the order of 100 to 200 microns from the LED array 42. A shim is usedbetween LED board assembly 12 and the proximal flange of lens housing 14to set the distance between the LED array 42 and input to the tapercollection optic 52 to account for manufacturing tolerances.

The glass elements 52, 54, 58, 64, and 16 in a preferred embodiment aremade out of a low UV absorption glass type such as fused silica, but canbe made of other low dispersion UV transparent glass materials such asBK7 or B270, crystalline materials, salts, diamond, sapphire or UVtransparent silicone or sol gels. Optimally, the materials areanti-reflection coated to minimize Fresnel reflective losses and tominimize ghosting at the illumination plane. The housing 14 is made ofblack anodized aluminum in a preferred embodiment due to its low mass,high thermal conductivity and reasonably low thermal expansioncoefficient and low cost. High flux UV energy can degrade standard blackanodization so more robust coatings, such as Optic Black™ manufacturedby Pioneer Metal Finishing of Green Bay, Wis., are preferably used.Standard anodization will turn color and result in deposits onto thelens elements which can result in loss of optical power. Additionally,the difference in thermal expansion coefficients between the opticmaterials and the housing and spacers are selected to minimizetemperature dependent changes in focus.

Referring now to FIG. 4A, there is shown a diagrammatic isometric view70 of the LED board assembly 12 of FIG. 3 as interfaced to collectionoptic 52. FIG. 4A affords a better view of anode connector 24 andcathode connector 26 as well as photosensor and thermistor connector 28.The thermistor 74 and photosensor 76 are shown to the upper left of theLED array 42. The thermistor allows a control system to continuallymonitor temperature in the event that the cooling system is turned offor fails. Such a control system quickly shuts off the LED die array inthe event of a sudden spike in temperature to avoid catastrophicfailure. An additional use of the thermistor is to enable a temperaturesensitive correction of the output by calibrating output of the LEDarray as a function of current and temperature. For example, if theambient temperature were to increase then the temperature dependentoutput of the LED array would decrease. A closed loop system can becontrolled by an analog or digital control loop to adjust the outputcurrent to maintain constant LED output power.

This approach of monitoring output with a thermistor, however, does notaccount for long term decreases in LED output with time. Therefore, aphotosensor 76, which is comprised of a UV sensitive detector, such as asilicon photodiode coupled to a transimpedance amplifier, is able tomonitor changes in output and drive the current higher through a closedloop control to maintain constant output power over both time andtemperature. The bottom cavity of the housing 14 has features that allowa small amount of the UV energy emitted by the LED array to be sampled.Alternatively, a separate photosensor off the LED board can be used tolook transverse to the Z-axis in the middle of the collection optic 52to sample the amount of UV energy transmitted by the system. Siliconphotosensors are very stable with time and the applicable temperaturerange and thus make an accurate measurement of optical power.

The taper 52 is comprised of a rectangular input side 78 mating withinless than 500 microns of the LED array 42 and with an aspect ratiosimilar to that of the DMD, although strictly speaking the aspect ratioof the output aperture 80 comprising long side 82 and small side 84 iswhat is imaged onto and determines the size of the DMD illumination. Ifthe aspect ratio in the two orthogonal directions of the input andoutput apertures are not the same then the far field is not symmetric,but assuming the aperture stop of the illumination system is circular,the resulting far field is circular so it is not a requirement that theaspect ratios of input and output of the taper be the same. Thus, theaspect ratio of output face 80 of tapered collection optic 52 ispreferably substantially similar to that of the DMD micromirror arraypositioned at illumination plane 18 of FIG. 1. The sides of the taperare substantially flat with larger face 86 and smaller face 88 symmetricon two opposite sides. The purpose of the taper is to capture the UVenergy emitted by LED array 42 which extends over a hemisphere in anglespace (2π steradians) and convert it into a smaller angle θ with respectto the optical Z-axis that is equal to or greater than the 12° halfangle required at the input of the lens system of FIG. 3 to assure thatthe lens system aperture stop 60 of FIG. 3 is fully filled, otherwisethe output would be reduced. As will be described in greater detailbelow, the output of the taper in a preferred embodiment overfills theaperture stop 60 by an amount that blocks approximately 22% of the powerincident on it. Additionally, since the DMD micromirrors are ditheredalong one plane, it is possible to limit the angular acceptance of raysin the mirror plane to the required 12° half angle, but increase theangle in the orthogonal axis to effectively increase the averagenumerical aperture of the system and thereby further increase intensityon the illumination plane. This approach requires the projection lenssystem to have an equivalently shaped elliptical (or rectangular)aperture stop to pass all the power exiting the DMD.

Also indicated in FIG. 4A are a pair of opposed holes 72 through the LEDboard copper substrate 22 which are interfaced to two kinematic pins,one round, one oval to allow the position of the taper input aperture 78to be accurately and reliably positioned relative to the LED array 42.These two holes 72 act as the datum feature to which the LED die arrayis aligned.

With reference now to FIG. 4B, a diagrammatic cross sectional view 90 ofa close up of the area near the LED array of FIG. 4A is shown. Asynthetic polycrystalline diamond heat spreader 92 is attached to LEDboard copper substrate 22 by use a high thermal conductivity solder. TheLED die array 42 is attached to gold plated traces on the top of thediamond heat spreader by use of a similar high thermal conductivitysubstantially void free solder. The LED die can be operated at a currentup to the order of 5 Amperes per square centimeter at a voltage on theorder of 5 Volts. That results in a heat flux on the order of 2,000W/cm2. The transverse thermal conductivity of diamond heat spreaders ison the order of 2000 W/m-K, which is roughly five times that of bulkcopper. Diamond heat spreaders are anisotropic so the thermalconductivity in the thin dimension (into the heat spreader Z-Axis) isless but still on the order of 600 W/m-K, which is still better thanbulk copper. Thus, the heat spreader acts to spread the heat out in theX-Y plane and thereby reduces the heat flux into the copper substrate22, which further spreads the heat before entering the water heatexchanger 30, of FIG. 4A. The diamond heat spreader 22 has gold traces94 to which wire bonds 96 are attached from the top of the LED die andin turn those traces are attached by a series of wire bonds to thecopper traces on top of the COB.

Reference is now made to FIG. 5A which shows a diagrammatic isometricview of the system of FIG. 4A with the tapered collection optic 52removed. The additional features on the board shown in this viewcomprise four symmetrically opposed holes 102 which allow the LED boardto be attached to the housing 14 prior to attachment of the water heatexchanger 30 of FIG. 2. FIG. 5B shows a diagrammatic isometric close upview of the LED die array 42 and heat spreader 92. There are three rowsof six each UV LED die closely packed together to form an eighteen diearray for use with a 1080p 0.95 in. diagonal Texas Instruments® DMDdevice. Another embodiment of the system uses the same lens system,housing and LED board but is designed for the 1024 by 768 by 0.70 in.diagonal Texas Instruments® DMD device and is comprised of aproportionally smaller taper and a three by four die array of UV LEDs.The typical LED die is approximately 1,000 microns square by about 100microns in thickness with two each wire bond pads per die. There are twosets of wire bond traces on top and bottom of the central trace wherethe LED die are attached on the diamond substrate 92. The two sets allowfor two different LED wavelength bins to be used to provide a broaderspectrum. This is useful for some types of photoinitiators that maychange their absorption spectrum throughout the photopolymerizationprocess. Some photopolymers are known to have surface cure inhibition byoxygen preventing good surface curing. The use of UV energy on the orderof 250 nm to 280 nm has been shown to prevent this. Such a system can beaccommodated with the approach detailed above making use of the recentlyreleased 280 nm LED die that are now commercially available. It will beobvious to those of skill in the art that the number of differentspectral bins can be extended up to the number of individual LED die ifdesired with appropriate features on the diamond heat spreader and LEDboard.

The diamond heat spreader 92 of FIG. 5B is expensive so it is importantfor overall cost and performance to determine how large the heatspreader should be. As the size of the heat spreader increases, it hasless incremental benefit. The relative size of the LED array shown inthe preferred embodiment of FIG. 5B is the order of 3.3 mm by 6.6 mm andthe size of the diamond heat spreader is 8.0 mm by 10.0 mm which wasfound to be an optimal tradeoff between heat spreader size, reduction inΔT between the back side of the LED board and the LED junctiontemperature and cost, resulting in a temperature drop on the order of20° C. to 25° C. relative to room temperature of 25° C. This significantdrop in temperature allows the LED die to be continuously operated atcurrent densities on the order of 5 Amperes per centimeter squared andsince they are at a lower temperature for a given intensity, they lastlonger. The lifetime of LED die is known to decrease exponentially withincreasing junction temperature due to temperature dependent diffusionprocesses within the diode junction. Computational Fluid Dynamics (CFD)software packages such as SolidWorks® Flow® can be used to runsimulations to determine optimal heat spreader size. Alternatively, thedata can be determined empirically.

The LED die array shown in FIG. 5B is attached on the bottom side in acommon anode configuration to allow the die to be butted togetherthereby maximizing the radiance. One down side of common anode is thatif all the cathodes are tied together to the same current source thendifferences in forward voltage between LED die can lead to differencesin current through each die. This in turn leads to differences intemperature dependent aging. Common anode prevents the LED die frombeing wired in series to assure that each die gets the same current. Inanother embodiment of the invention, each LED die is wire bonded to anindependent current source. In this way, it is possible to get thethermal and tight packing advantages as well as precise control of eachLED die. Additionally, such a current driving scheme is better thanseries connection since the failure of one LED die does not affect theother die as in a series connection. A further advantage of driving eachLED die is that the forward voltage of each can be monitored with timeand the system can go through a calibration at start up to monitor theoutput of each LED die with time to predict lifetime and inform theoperator of any future maintenance that may be required.

With reference now to FIG. 6A, a diagrammatic top view 120 of the systemof FIG. 1 is shown with the housing, spacers, and taper holdercomponents removed for clarity. Lines 124 emanating from the outputaperture 80 of tapered collection optic 52 and lines 130 converging fromlens 16, respectively, are shown to indicate the optical ray paths asimaged between the taper output aperture and the DMD micromirror surface136. Rays exiting from a given point on the output of the taper surface80 converge toward a point of rays 134 on the DMD with the chief raynormal to the DMD surface in a telecentric condition as required forproper use of the DMD. A block of glass 128 is shown positioned betweenthe last lens element 16 of the illumination system and the glass coverplate 132 of the DMD to represent the optical effect (unfolded path) ofa total internally reflecting (TIR) coupling prism. This type of prismis often used to couple the illumination into the DMD at an angle ofincidence on the order of 24°. For systems that do not incorporate a TIRprism and therefore have a greater angle of incidence on the DMD, therecan be a more significant keystone effect due to entering and or exitingthe DMD at a larger angle relative to the normal to its surface. Such aTIR prism is used in a preferred embodiment of a 0.95 in. diagonal UVilluminator shown here, however, it will be recognized by those of skillin the art, that the invention could be used without a TIR prism. Theoptical design is optimized with a piece of glass representing the prismto take proper account of and correct for the aberrations, however, theprism is not included as part of the illumination housing. Rays emittedfrom aperture 80 but outside the angle space of the lens systemsaperture stop 126 are absorbed by the stop and are prevented fromtransmitting toward the DMD 136. The system of lenses 54, 58, 64, and 16are designed to be telecentric in both object (taper output aperture)and image (DMD micromirror plane) space. On the object side,telecentricity approximates the virtual far field of the taper, which isalso a function of taper length.

In the case where no TIR prism is used to couple the UV energy to theDMD, it is possible to correct for increased keystone by adding arectangular cross section hollow end piece as an extension to the taper.This hollow end piece directly butts the taper on the proximal side andhas a tilted plane in the opposite direction of the tilt of the DMDrelative to the optical Z-Axis of the illumination system of FIG. 1.This method of imaging a tilted plane can be used to compensate andsubstantially correct for increased keystone resulting from directimaging onto the DMD without a TIR prism. It is critical that the shorthollow internally reflective walled device be used at the end of thetaper instead of just tilting the output face of the taper, since doingso results in refraction and therefore steering of the chief rays.

The taper 52 of FIG. 6A is designed with a far field distribution whichoverfills the aperture stop 126 of the UV illumination lens system toincrease radiance. This is accomplished by trading off efficiency byrecognizing that rays at smaller emitted angles from the LED surfacehave less power as emitted from a Lambertian source due to the decreasein projected area with increasing angle, which is the well know cosine θeffect. Therefore, by increasing the size and thereby, the Etendue ofthe LED array 42 and purposely rejecting high angular extent raysemitted by the LED array 42 as they are absorbed by the optical stop 126of the lens system, the radiance on the DMD 136 can be increased fromwhat it would be limited to by a purely Etendue conserving design. Thissystem can, however, work with a smaller taper and LED array designed toconserve Etendue and achieve high spatial uniformity, but at a lowerradiance since the fewer LED die that are required for Etendueconserving designs are proportionally smaller and therefore operate athigher current density and lower efficiency relative to driving a largerarray. This concept of trading off efficiency for increased radiance ishighly beneficial. The water heat exchanger 30 has input and outputports (reversible) 122 that are attachable to a water to air heatexchanger by use of tubing clamped to hose barbs on 122.

FIG. 6B shows a diagrammatic side view 120 of the system of FIG. 6Arepresenting the narrow output dimension of the taper 52 and DMD 136.Likewise collectively rays 138 that are emitted at angles larger thanthe aperture stop 126 are absorbed by it and prevented from transmittingto the DMD. It should be noted that in both FIGS. 6A and 6B, only thoserays that pass through the aperture stop 126 are shown. Rays at higherangles that are emitted by the taper 52 and overfill the aperture stop126 are not shown.

With reference now to FIG. 7A, an alternative embodiment 140 of atapered collection optic is show in diagrammatic isometric view. Theinput aperture 142 has long side 144 and short side 146 with asubstantially rectangular aperture. Short side 150 and long side 148 areshown corresponding to short output edge 158 and long output edge 160,respectively. As was the case for the taper described above, all sixsides of the taper are polished to reduce or minimize surface scatteringand are generally made of glass such as fused silica, UV grade lowfluorescence Schott BK7 or B270 glass or equivalent. UV rays reflectmultiple times as they progress down the taper by total internalreflection. This is a much more efficient reflection means in comparisonto reflective hollow tapers, which suffer significant cumulativereflective losses, particularly in the UV where it is more difficult toachieve a high reflectivity coating. The input aperture dimensions ofthe taper 140 are identical to that of taper 52 as they are bothdesigned to interface to the same LED array 42; however, the outputdimensions are proportionally larger. The output dimensions for taper140 are chosen such that the angular extent of the taper output matchesthat required to fill the aperture stop of the system of FIGS. 1, 2, 3,and 6. This contrasts to the taper 52 which was specifically designed tooverfill the lens aperture stop and take advantage of the cosine θeffect as described above for trading off efficiency for intensity. Bychanging to the configuration of taper system 140 with a matched farfield and by recovering the UV energy that reflects back down the taperoff of a high reflectance mirror coating 162, the UV energy that isotherwise absorbed by the aperture stop 126 is partially recovered.

FIG. 7B shows a diagrammatic top view of the taper 140 of FIG. 7A. Ray159 emitted by the LED array and passing through input aperture 142reflects off the sides of the taper 140 by total internal reflection andimpinges on internally mirrored surface 162 as indicated by ray 168.When ray 168 strikes the LED array, it is diffusely scattered backtoward the taper with most of the energy passing back out of aperture152 and is thereby recovered and can result in an increase by 15% ormore in the intensity at the illumination plane. Most of the rays 157like those recovered rays just described pass directly out of theaperture 152. It will be clear to those of skill in the art that furtherincreases in intensity can be achieved by increasing the LED array size,input and output apertures of the taper, but maintaining the same exitaperture size 152. There are diminishing returns; however, as theincreased heat load from the array can reduce the output per LED therebydecreasing the benefit. In addition, the larger the area of the mirroredsurface 162 relative to the exit aperture 152, the lower the efficiencydue to finite reflective losses of the mirror and LED surfaces as wellas losses out the gap between the input aperture 142 of the taper andthe LED array 42.

FIG. 7C shows a diagrammatic side view of the narrow dimension of taper140 of FIG. 7A. The same effect is true for this view for rays strikingthe mirror surface 162 and returning down the length of the taper,reflecting off the LED array and exiting out of aperture 152. Some ofthe light that is reflected back off the LED array will be incident onthe mirror 162 again. The more times this happens, the more loss occursfor such rays, which again is why there are diminishing returns as themirrored area size approaches that of the emitting aperture area.

Other shapes that can be used for the non-imaging collection opticinclude compound parabolic concentrators (CPCs). The system ischaracterized by the requirement to have high near field uniformity atthe output of the collection optic. Non-imaging straight walledcollection optics with an even number of sides are excellent atproducing very high near field uniformity at their output. CPCs workwell as concentrators (or collectors in reverse), however; a CPC onlyresults in uniform near field output at its exit aperture if the inputaperture is uniformly filled. That precludes the use of more than onespectral bin of LED die as can be accommodated by embodiments describedabove. Furthermore, unless a mold process is used, glass CPCs are moredifficult to make than tapers which can be conventionally polished. Itis possible to substitute a rectangular four sided CPC in this systemfor the taper assuming only a single wavelength bin was used, however,even with a single wavelength bin LED die array, the uniformity achievedwith a taper is generally better than that achieved with a CPC of anequivalent length.

FIG. 8A is a diagrammatic, cross sectional view of another embodiment ofthe UV LED DMD illumination source system designated generally as system160. The UV LED illumination source system 160 comprises an LED array161, a tapered non-imaging collection optic 162 with input face 163 andoutput face 164, a hollow reflective light integrator 165 with tiltedoutput face 166 to correct for image keystone on the DMD, a lens systemcomprising lens elements 167, 168, 169, 171, 172 and 173 and imagingrays 177 that is telecentric in both object and image space as well asat the aperture stop 170, a total internal reflection (TIR) prismelement 174 that represents a thick window with non-parallel input andoutput faces and a DMD element with window 175 and active area 176.

Another embodiment uses a TIR prism pair, represented in FIG. 8A as asingle component 174, and results in the chief ray of the illuminationsystem incident on the DMD surface 176 at an angle of 24 degrees. Thisallows the reflected beam off the tilted micro-mirrors of the DMD 176 toexit with the chief rays for any image point to be normal to the DMDactive area 176, thereby resulting in the least image distortion of theimage resulting from the projection lens. The result of the illuminationbeing imaged onto the DMD at 24 degrees angle of incidence for the chiefrays is that the illumination image is distorted. If this distortion isnot corrected, the object size must be increased to compensate for thisimage blur in order to achieve highly uniform intensity on the DMDactive area 176. The optical power that strikes the DMD outside itsactive area is partially absorbed by the DMD resulting in increasedheating of the DMD and therefore reduced temperature-dependent lifetime.Additionally, the optical power that is outside the active area of theDMD that is not absorbed results in scattered optical power that reducesthe image contrast resulting in compromised overall system performance.The illumination image is slightly oversized to address finite systemalignment tolerances, but the smaller the overfill area the better.

FIG. 8B is a diagrammatic, cross sectional view of the system of FIG. 8Awithout the imaged rays 177 shown. A reflective aperture stop 178 isshown with its first surface in the plane of and centered on the systemaperture stop 170. The optical system is configured such that the rayswhich are outside the dimensions of the aperture stop 170, that is,outside the transmissive central region, are reflected from thesurrounding reflective region back towards their source. Further, thelenses 167, 168 and 169 are configured such that reflections outside theaperture stop 170 are reflected off annular mirror 178 and imaged backinto the output aperture at the output face 166 of the hollow lightintegrator 165. This reflected light is subsequently reflected by TIRoff the walls of the tapered non-imaging collection optic 162 and thenincident on the LED array 161. This constrains the lenses between theoutput face of the hollow light integrator 165 and the reflectiveaperture stop to comprise one half of a symmetric one to one imaginglens system, otherwise the rays do not substantially image back to thehollow light integrator aperture and would be absorbed.

The diffuse reflectivity of UV LED die ranges from the order of 50% to70%, so that at least a portion of the UV light that reaches the LED diethat was returned from the reflective aperture stop has an opportunityto propagate back through the tapered non-imaging collection optic andaperture stop, and to be directed to the DMD active area. This effect isindicated by ray 179 leaving the LED die array 161, traveling throughthe non-imaging tapered collection optic 162, out of the hollow lightintegrator 165 through lenses 167, 168 and 169, reflecting offreflective aperture stop 178, at which time the ray is referred to asray 180, returning back through the hollow light integrator 165 andtapered collection optic 162 to the LED die where the ray is diffuselyscattered and returns as ray 181 through the aperture stop 178 and ontothe active are of the DMD 176.

One benefit from this reflective telecentric re-imaging aperture stopand lens system is the ability to reclaim optical power that otherwiseis stopped by the aperture stop thereby increasing intensity on the DMDactive area 176 by the order of 10% or more. Thus, overall systemefficiency is improved. This additional intensity can be used to realizefaster cure times or used to achieve the same intensity at lowerelectrical currents and therefore lower LED array junction temperatures,increase LED life, or both. A further benefit is that if the aperturestop is not reflective, the absorbed optical power would increase theheat load and possibly require a cooling system to remove the heat,thereby increasing system cost and complexity.

FIG. 8C is an image showing the intensity that is incident outside theoptical aperture stop of the system 160 of FIGS. 8A and 8B. This opticalpower outside the aperture stop is the result of using the non-imagingcollection optic 162 for which there is some light resulting outside thefar field angle of the aperture stop due to its finite length. For aninfinitely long tapered collection optic, the amount of light outsidethe aperture stop is minimal, however, for finite and practical lengthtapered collection optics, this higher angle far field light results.The system of FIGS. 8A and 8B reclaims this optical power.

FIG. 9A is a diagrammatic cross sectional view of a system 190 similarto the system 160 of FIGS. 8A and 8B, but with the taper rotated by 45degrees to properly image onto the DMD surface 176 since the DMD needsto be oriented at 45 degrees in the plane of the mirror tilt axis.Additionally, TIR prism elements 191 and 192 replace component 174 ofFIGS. 8A and 8B to better represent how the light totally internallyreflects off of the transverse face of element 192 toward the projectionlens 194. Optical energy reflected from the DMD returns to the surfaceat the interface of TIR prism elements 191 and 192 and is reflectedupward in the figure through projection lens elements 194 to form animage at an image plane 195. The chief ray normal to and reflected offthe DMD, total internal reflected off the transverse face of element 192and passing through projection lens elements 194 is represented by ray193 which is imaged from the DMD to projection image plane 195. The tiltof the hollow light integrator 165 reduces the image blur that occursdue to the light being incident on the DMD 176 at an angle of 24degrees. In a preferred embodiment this angle is 6 degrees relative tothe normal to the optical axis or, stated another way, relative to theangle of the output face of the tapered collection optic 162. The outputface is in contact with the input face of the hollow light integrator165. The hollow light integrator 165 addresses new applications for theDMD illumination system including 3-D digital printing with UV curablematerials. For applications such as maskless lithography theillumination system is scanned over the printed circuit board such thateach point on the circuit board is illuminated by all pixels along thescan direction. 3-D digital printing applications, however, are nottypically scanned, thus each point in the image is only illuminated by agiven pixel on the DMD. Thus, if there are any imperfections on theoptical components near the object plane such as dust, coating or glassimperfections, typically on the output face of the tapered collectionoptic and perhaps a window or lens in contact or near contact to theoutput face of the tapered collection optic, those imperfections canresult in image artifacts. As a result, curing for some portions of theillumination area may be compromised. Using the hollow light integrator165 with inside reflective walls, the object plane (i.e., the outputface 166) that is imaged onto the DMD plane is in air and within theclosed illumination system such that no imperfections related to dust orcoating defects are present at the object plane. The same situationoccurs for other fixed illumination plane applications, includingmicroscopy, for which a hollow light integrator is similarly used.Hollow light integrators are typically used for lower performanceoptical configurations that do not use the more optimal tapered glassnon-imaging collection optics. Such configurations inherently do nothave the issues with defects at the object surface that can occur forhigher performance configurations employing tapered non-imagingcollection optics.

FIG. 9B is a diagrammatic isometric view of the tapered non-imagingcollection optic 162 and the hollow light integrator 165. The opticalpower density incident on and passing through the optical systems 160and 190 of FIGS. 8A, 8B and 9A is high so materials such as fused silicathat do not solarize with extended exposure to high flux UVA light areused. Standard high performance glasses such as N-BK7 have sufficientlylow absorption initially, but the absorption band shifts to longerwavelengths due to exposure to UVA light making them unsuitable as suchglasses may overheat by UV light absorption and compromise intensity.The tapered collection optic 162 has a ratio of a short input side 201to a longer input side 202 that generally corresponds approximately to aratio of the short and long sides of the DMD active area. Some deviationfrom this ratio is allowed to accommodate realistic LED die array sizes.The output face of the taper 162 is coincident with and slightly largerthan the hollow aperture of the hollow light integrator 165. The longside 205 and the short side 204 of the hollow aperture correspond to theobject size to properly image onto and slightly overfill the active areaof the DMD, typically by the order of five percent. The overfillaccommodates finite mechanical tolerances to ensure high uniformity ofintensity over the entire active area of the DMD which is subsequentlyimaged by the projection lens elements 194 onto the curing plane 195.The length 203 of the hollow light integrator 165 is sufficiently largeto ensure that any imperfections on the taper output face aresufficiently blurred by the time they reach the output face 166 which isthe object plane that is imaged onto the DMD. In a preferred embodimentthe length 203 of the hollow light integrator 165 is the order of 10 mm,driven primarily by mechanical holding requirements. The length 203 ofthe hollow light integrator 165 is generally limited to minimize overallsystem length and to decrease losses due to the finite reflectivity ofthe inside walls. In a preferred embodiment the reflectivity of theinside walls is greater than ninety-six percent.

FIG. 10A is a diagrammatic isometric view of the hollow light integrator165 of FIGS. 8A, 8B, 9A and 9B. The hollow light integrator 165 can bemade from four pieces of fused silica glass with reflective coatings onthe inside surfaces that are secured together to define a hollow body.An adhesive material such as GE 3145 Silicone RTV that is not degradedby high flux UV exposure can be used. Alternatively, each of the fourreflective walls of the hollow body can be made from a reflectivemirrored metal sheet. For example, the mirrored metal sheeting can be a0.020 inch thick Anolux MIRO IV metal sheet (available from Anomet, Inc.of Brampton, Ontario, Canada) having a reflectivity of greater thanninety five percent in the UVA spectrum. There is also a reflectivecoating 212 which reflects incident UV light over the overlap regionwhere the rectangular output face of the tapered collection optic 162 ofFIG. 9B is in contact with the input face 211 of the hollow lightintegrator 165. The long output face 213 and short output face 214 liein the plane of the output face 166 which is imaged onto the DMD. Theangular tilt between input face 211 and output face 166 for a preferredembodiment is the order of 6 degrees with reference to FIG. 10B. FIGS.10C and 10D show the input face 211 with reflective area 212 and theoutput face 166, respectively.

FIG. 10E shows a diagrammatic isometric view of a hollow lightintegrator 220 similar to the hollow light integrator 165 of FIG. 10A,but with an output face 225 that is parallel to the input face 221. Areflective input aperture 222 acts in a like manner to the reflectivearea 212 in FIG. 10A. Likewise, the reflective input aperture 222includes a long side 223 and short side 224 corresponding to the longand short side of the image of this plane on the DMD active area 176 ofFIGS. 8A, 8B and 9A. FIG. 10F illustrates how the output face 225 isparallel to the input face 221. The input and output faces 221 and 225are shown in bottom and top diagrammatic views in FIGS. 10G and 10H,respectively.

FIG. 11A shows a hollow light integrator 220 in top view and side viewand the resulting image on the DMD plane is shown in FIG. 11B. Thehollow light integrator 220 is not tilted, that is, the input face andoutput faces are parallel, therefore the resulting image shown in FIG.11B exhibits image blur along two opposite corners because the imageplane is tilted at 45 degrees relative to the sides of the imageaccording to the tilt plane of the DMD mirrors. This blurred image meansthat the object size (corresponding to dimensions of sides 223 and 224of FIG. 10E) has to be increased to assure the full DMD active area isuniform in intensity. In contrast, the tilted face of the hollow lightintegrator 165 shown in FIG. 11C results in a keystone corrected imageas shown in FIG. 11D that is substantially in focus over the entireimage. The small image blur in the middle of the long sides of the imageis due to field curvature of the illumination lens system, which is oneof the tradeoffs made to minimize the number of elements and to keepsystem cost lower while assuring that telecentricity conditions are met.The image of FIG. 11D is a parallelogram, that is, the left and rightsmaller sides are vertical; however, the top and bottom sides are nothorizontal. This results in optical power outside the active area of theDMD. The optical power outside the active area leads to increasedheating and therefore reduced lifetime of the DMD as well as increasedstray light resulting in compromised image contrast. Consequently, apartial cure may occur in areas that should be dark, therebycompromising the UV curing system performance. Thus, in a preferredembodiment a field stop 231 is located at the tilted output face of thehollow light integrator 230 shown in FIG. 11E. The non-perpendicularadjacent sides of the parallelogram-shaped field stop 231, with shortvertical sides 232 and tilted adjacent sides 233, act to cut off thelight producing the parallelogram at the DMD surface, thereby resultingin a sharply focused rectangular illumination area image as shown inFIG. 11F. The field stop 231 optimizes system performance by reducingthe thermal load on the DMD, thereby increasing the operational lifetimeof the DMD and reduces stray light for improved image contrast.

What is claimed is:
 1. An optical system for a digital micromirrordevice illuminator, comprising: a light emitting diode (LED) array; atapered non-imaging collection optic having an input aperture in opticalcommunication with the LED array to receive optical energy emitted bythe LED array and having an output aperture; a reflective aperture stop;and a telecentric lens system disposed between the tapered non-imagingcollection optic and the reflective aperture stop along an optical axisdefined between the LED array and the reflective aperture stop, thetelecentric lens system configured as a first half of a symmetric one toone imager for an object plane on the optical axis and as a second halfof the symmetric one to one imager for optical energy reflected from thereflective aperture stop toward an image plane on the optical axis,wherein the reflective aperture stop provides reflected optical energythat propagates from the output aperture of the tapered non-imagingcollection optic through the input aperture of the tapered non-imagingcollection optic and is incident at the LED array.
 2. The optical systemof claim 1 wherein the object plane and the image plane are coincidenton the optical axis.
 3. The optical system of claim 2 wherein the objectplane and the image plane are located at the output aperture of thetapered non-imaging collection optic.
 4. The optical system of claim 1wherein the reflected optical energy incident at the LED array isdiffusely reflected and wherein at least a portion of the diffuselyreflected optical energy propagates from the input aperture of thetapered non-imaging collection optic to the output aperture of thetapered non-imaging collection optic.
 5. The optical system of claim 1further comprising a hollow light integrator having an input face withan input aperture therein and an output face with an output aperturetherein, the input aperture of the hollow light integrator beingadjacent to the output aperture of the tapered non-imaging collectionoptic.
 6. The optical system of claim 5 wherein the input face and theoutput face of the hollow light integrator are tilted with respect toeach other.
 7. The optical system of claim 5 wherein the object planeand the image plane are coincident on the optical axis and located atthe output aperture of the hollow light integrator.
 8. The opticalsystem of claim 1 wherein the LED array is an ultraviolet LED array. 9.The optical system of claim 1 wherein the reflective aperture stopcomprises a transmissive central region surrounded by a reflective outerregion.
 10. The optical system of claim 5 wherein the taperednon-imaging collection optic comprises an output face comprising theoutput aperture of the tapered non-imaging collection optic and whereinthe output face of the tapered non-imaging collection optic is largerthan the input aperture of the hollow light integrator.